BACKGROUND
At Epworth Radiation Oncology (ERO), Ultrasound (US) is employed for localisation of target anatomy and Organs At Risk that require delineation in Brachytherapy (BT) treatment planning. US is utilized in both Low Dose Rate and High Dose Rate Brachytherapy for prostate cancer and its application extends to real time guided cervix Brachytherapy and dosimetry. While BT US compliance is not mandatory in Australia, it is fundamental to best practice and has been implemented at ERO.
PURPOSE
ERO aims to strengthen its BT service through the implementation of a US Quality Assurance (QA) program that follows the recommendations from the American Association of Physics in Medicine Task Group 128: Quality Assurance Tests for Prostate Brachytherapy Ultrasound Systems (AAPM TG 128)3 using the CIRS 045 Brachytherapy QA Phantom (CIRS 045) (Fig. 1).
The AAPM TG 128 recommendations were also performed in tap water and saline (0.9% w/v sodium chloride) phantoms with different acoustic properties compared to soft tissue, to test which recommendations can be validated and compared to the performance of CIRS 045.
METHOD
AAPM TG 128 recommendations were tested using a BK Medical FlexFocus 800 ultrasound scanner, BK Medical Endocavity Biplane 8848 transducer attached to a CIVCO EX3 Stepper, the CIRS 045 and ultrasound gel for coupling.
Using 5 x 18 gauge needles inserted through 2 x Template Grids for stabilization (Fig. 2), this configuration was submerged in the tap water and saline phantoms to perform the AAPM TG 128 tests. The lowest gain setting was used to minimise lateral and axial needle blurring1.
Measurements were taken from the brightest (highest echo amplitude) portion of the first image blur pattern and repeated 5 times to establish baselines. DISCUSSION
The CIRS 045 made of Zerdine, with an acoustic velocity of 1540m/s matching that of soft tissue, performed all test recommendations with sub millimetre accuracy and consistency (Table 2).
Without high contrast volumes with known geometries, the liquid phantoms could not register area, volume and planning system test measurements. The lack of contrast of the US in water meant depth of penetration could not be tested. As a result, the liquid phantoms could only perform 3 of the 7 recommendations (Table 1).
The results measured from the 3 recommendations tested in the liquid phantoms could not match the performance of the CIRS 045. However the saline phantom gave more accurate results than the tap water phantom.
The different acoustic properties to that of soft tissue resulted in varied focusing of the ultrasound beam2, giving way to greater variation in test measurements, most notable in the distance measurement tests (Table 2). With a greater acoustic velocity, the saline had increased performance over the tap water phantom, which did not meet baseline action level standards. This is consistent with Pfeiffer, Sutlieg, Feng, Pierce and Kofler recommendations for phantom solutions3.
The differing acoustic properties also meant the water and saline phantoms could not reliably assess axial and lateral resolution. Nevertheless, once baseline measures have been recorded, the liquid phantoms can still be used to test for consistency i.e., precision rather than accuracy even in these properties2.
CONCLUSION
US quality control testing in Brachytherapy is not mandatory in Australia. However, to strengthen the quality of our Brachytherapy service, ERO have implemented CIRS 045 Brachytherapy QA Phantom which performs all AAPM TG 128 test recommendations. This QA will be conducted bi-annually to ensure results fall within action levels of the benchmark measurements presented.
When a purpose built phantom is not available, one using saline is more reliable option than a phantom of tap water in performing US QA, however neither phantoms meet all AAPM TG 128 recommendations.